The present invention relates to the field of agonists for the family of nuclear receptors identified as peroxisome proliferator-activated receptors.
The following description is provided solely to assist the understanding of the reader. None of the references cited or information provided is admitted to be prior art to the present invention. Each of the references cited herein is incorporated by reference in its entirety, to the same extent as if each reference were individually indicated to be incorporated herein in its entirety.
The peroxisome proliferator-activated receptors (PPARs) form a subfamily in the nuclear receptor superfamily. Three isoforms, encoded by separate genes, have been identified thus far: PPARγ, PPARα, and PPARδ.
There are two PPARγ isoforms expressed at the protein level in mouse and human, γ1 and γ2. They differ only in that the latter has 30 additional amino acids at its N terminus due to differential promoter usage within the same gene, and subsequent alternative RNA processing. PPARγ2 is expressed primarily in adipose tissue, while PPARγ1 is expressed in a broad range of tissues.
Murine PPARα was the first member of this nuclear receptor subclass to be cloned; it has since been cloned from humans. PPARα is expressed in numerous metabolically active tissues, including liver, kidney, heart, skeletal muscle, and brown fat. It is also present in monocytes, vascular endothelium, and vascular smooth muscle cells. Activation of PPARα induces hepatic peroxisome proliferation, hepatomegaly, and hepatocarcinogenesis in rodents. These toxic effects are lost in humans, although the same compounds activate PPARα across species.
Human PPARδ was cloned in the early 1990s and subsequently cloned from rodents. PPARδ is expressed in a wide range of tissues and cells with the highest levels of expression found in digestive tract, heart, kidney, liver, adipose, and brain. Thus far, no PPARδ-specific gene targets have been identified.
The PPARs are ligand-dependent transcription factors that regulate target gene expression by binding to specific peroxisome proliferator response elements (PPREs) in enhancer sites of regulated genes. PPARs possess a modular structure composed of functional domains that include a DNA binding domain (DBD) and a ligand binding domain (LBD). The DBD specifically binds PPREs in the regulatory region of PPAR-responsive genes. The DBD, located in the C-terminal half of the receptor contains the ligand-dependent activation domain, AF-2. Each receptor binds to its PPRE as a heterodimer with a retinoid X receptor (RXR). Upon binding an agonist, the conformation of a PPAR is altered and stabilized such that a binding cleft, made up in part of the AF-2 domain, is created and recruitment of transcriptional coactivators occurs. Coactivators augment the ability of nuclear receptors to initiate the transcription process. The result of the agonist-induced PPAR-coactivator interaction at the PPRE is an increase in gene transcription. Downregulation of gene expression by PPARs appears to occur through indirect mechanisms. (Bergen & Wagner, 2002, Diabetes Tech. & Ther., 4:163–174).
The first cloning of a PPAR (PPARα) occurred in the course of the search for the molecular target of rodent hepatic peroxisome proliferating agents. Since then, numerous fatty acids and their derivatives including a variety of eicosanoids and prostaglandins have been shown to serve as ligands of the PPARs. Thus, these receptors may play a central role in the sensing of nutrient levels and in the modulation of their metabolism. In addition, PPARs are the primary targets of selected classes of synthetic compounds that have been used in the successful treatment of diabetes and dyslipidemia. As such, an understanding of the molecular and physiological characteristics of these receptors has become extremely important to the development and utilization of drugs used to treat metabolic disorders. In addition, due to the great interest within the research community, a wide range of additional roles for the PPARs have been discovered; PPARα and PPARγ may play a role in a wide range of events involving the vasculature, including atherosclerotic plaque formation and stability, thrombosis, vascular tone, angio-genesis, and cancer.
Among the synthetic ligands identified for PPARs are Thiazolidinediones (TZDs). These compounds were originally developed on the basis of their insulin-sensitizing effects in animal pharmacology studies. Subsequently, it was found that TZDs induced adipocyte differentiation and increased expression of adipocyte genes, including the adipocyte fatty acid-binding protein aP2. Independently, it was discovered that PPARγ interacted with a regulatory element of the aP2 gene that controlled its adipocyte-specific expression. On the basis of these seminal observations, experiments were performed that determined that TZDs were PPARγ ligands and agonists and demonstrate a definite correlation between their in vitro PPARγ activities and their in vivo insulin-sensitizing actions. (Bergen & Wagner, 2002, Diabetes Tech. & Ther., 4:163–174).
Several TZDs, including troglitazone, rosiglitazone, and pioglitazone, have insulin-sensitizing and anti-diabetic activity in humans with type 2 diabetes and impaired glucose tolerance. Farglitazar is a very potent non-TZD PPAR-γ-selective agonist that was recently shown to have antidiabetic as well as lipid-altering efficacy in humans. In addition to these potent PPARγ ligands, a subset of the non-steroidal antiinflammatory drugs (NSAIDs), including indomethacin, fenoprofen, and ibuprofen, have displayed weak PPARγ and PPARα activities. (Bergen & Wagner, 2002, Diabetes Tech. & Ther., 4:163–174).
The fibrates, amphipathic carboxylic acids that have been proven useful in the treatment of hypertriglyceridemia, are PPARα ligands. The prototypical member of this compound class, clofibrate, was developed prior to the identification of PPARs, using in vivo assays in rodents to assess lipid-lowering efficacy. (Bergen & Wagner, 2002, Diabetes Tech. & Ther., 4:163–174).
Fu et al., Nature, 2003, 425:9093, demonstrated that the PPARα binding compound, oleylethanolamide, produces satiety and reduces body weight gain in mice.
Clofibrate and fenofibrate have been shown to activate PPARα with a 10-fold selectivity over PPARγ. Bezafibrate acted as a pan-agonist that showed similar potency on all three PPAR isoforms. Wy-14643, the 2-arylthioacetic acid analogue of clofibrate, was a potent murine PPARα agonist as well as a weak PPARγ agonist. In humans, all of the fibrates must be used at high doses (200–1,200 mg/day) to achieve efficacious lipid-lowering activity.
TZDs and non-TZDs have also been identified that are dual PPARγ/α agonists. By virtue of the additional PPARα agonist activity, this class of compounds has potent lipid-altering efficacy in addition to antihyperglycemic activity in animal models of diabetes and lipid disorders. KRP-297 is an example of a TZD dual PPARγ/α agonist (Fajas, 1997, J. Biol. Chem., 272:18779–18789) DRF-2725 and AZ-242 are non-TZD dual PPARγ/α agonists. (Lohray, et al., 2001, J. Med. Chem., 44:2675–2678; Cronet, et al., 2001, Structure (Camb.) 9:699–706).
In order to define the physiological role of PPARδ, efforts have been made to develop novel compounds that activate this receptor in a selective manner. Amongst the α-substituted carboxylic acids previously described, the potent PPARδ ligand L-165041 demonstrated approximately 30-fold agonist selectivity for this receptor over PPARγ; it was inactive on murine PPARα (Liebowitz, et al., 2000, FEBS Lett., 473:333–336). This compound was found to increase high-density lipoprotein levels in rodents. It was also reported that GW501516 was a potent, highly-selective PPARδ agonist that produced beneficial changes in serum lipid parameters in obese, insulin-resistant rhesus monkeys. (Oliver et al., 2001, Proc. Natl. Acad. Sci., 98:5306–5311).
In addition to the compounds above, certain thiazole derivatives active on PPARs have been described. (Cadilla et al., Internat. Appl. PCT/US01/149320, Internat. Publ. W) 02/062774, incorporated herein by reference in its entirety.)
Some tricyclic-α-alkyloxyphenylpropionic acids were described as dual PPARα/γ agonists. Sauerberg et al., 2002, J. Med. Chem. 45:789–804.)
A group of compounds that were stated to have equal activity on PPARα/γ/δ was described in Morgensen et al., 2002, Bioorg. & Med Chem. Lett. 13:257–260.
Oliver et al., described a selective PPARδ agonist that promotes reverse cholesterol transport. (Oliver et al., 2001, PNAS 98:5306–5311.)
Yamamoto et al., U.S. Pat. No. 3,489,767 describes “1-(phenylsulfonyl)-indolyl aliphatic acid derivatives” that are stated to have “antiphlogistic, analgesic and antipyretic actions.” (Col. 1, lines 16–19.)
Kato et al., European patent application 94101551.3, Publication No. 0 610 793 A1, describes the use of 3-(5-methoxy-1-p-toluenesulfonylindol-3-yl)propionic acid (page 6) and 1-(2,3,6-triisopropylphenylsulfonyl)-indole-3-propionic acid (page 9) as intermediates in the synthesis of particular tetracyclic morpholine derivatives.